Specialty beverages in which at least a portion is made up of frothed or heated milk are becoming more and more popular. The best-known beverage of this type is a coffee of the cappuccino type. It comprises a liquid portion consisting of coffee topped by a layer of frothed milk which, because of its very much lower density, floats atop the surface of the liquid. In general, preparing one takes time, manipulation operations and cleaning.
The most customary way of preparing a milk-based froth is to pour the desired amount of milk into the container, immerse a steam outlet pipe from a coffee machine in the container, agitating it up and down to introduce the air needed to form the froth.
There are also various devices that include a heating and stirring arrangements to heat milk and incorporate air into the milk for frothing the milk. Such devices or similar devices are disclosed in U.S. Pat. No. 2,932,493, U.S. Pat. No. 3,356,349, U.S. Pat. No. 4,537,332, U.S. Pat. No. 4,620,953, U.S. Pat. No. 6,712,497, U.S. Pat. No. 6,318,247, WO 01/26520, WO 2004/043213, WO 2006/050900, WO 2008/046837, WO 2008/142154, WO 2009/074555, DE 1 131 372, DE 196 24 648, DE 89 15 094, EP 1 725 124, EP 1 746 920, EP 1 878 368.
It is also known to use a steam generator, with or without a mechanical stirring device to heat and incorporate fine bubbles into milk for frothing. Examples of such technology are disclosed in EP 1 223 839, EP 1 716 796, EP 1 733 663, EP 2 047 779, EP 2 098 145, 2 153 759, WO 2010/023312 and WO 2010/023313.
It is known to froth milk by injecting pulsed compressed air and vapour into a body of milk, as disclosed in EP 1 776 905. Other compressed air injection systems are disclosed in EP 2 070 455 and DE 44 45 436.
Furthermore, there are milk frothing devices that are arranged to force milk through small-sized channels, as disclosed in DE 197 19 784. Similar principles are disclosed in U.S. Pat. No. 4,144,293.
There is still a need to provide a simple system for generating high quality milk froth.